FIG. 1 illustrates a conventional current control system 100. As shown in FIG. 1, a DC/DC (direct current to direct current) converter 102 converts input power 122 to a charging current I110 for charging a battery pack 120. A current sensor 104 senses the charging current I110 and generates a sense voltage V118 that is proportional to the charging current I110.
More specifically, the current sensor 104 includes a sense resistor 110 coupled between the DC/DC converter 102 and the battery pack 120. The charging current I110 flows through the sense resistor 110, such that a voltage V110 on the resistor 110 can be given by V110=I110*R110, where R110 represents resistance of the resistor 110. An operational amplifier 112 in the current sensor 104 has a first input terminal 124 coupled to a terminal of the resistor 110, and a second input terminal 126 coupled to another terminal of the resistor 110 via a resistor 114. Since voltage levels at the first and second input terminals 124 and 126 are substantially the same, a voltage V114 on the resistor 114 can be substantially equal to the voltage V110 on the resistor 110, e.g., V114=V110=I110*R110. Furthermore, an output signal 128 of the operational amplifier 112 can turn on a switch 116 that is coupled to the second input terminal 126 and ground via a resistor 118. As such, a current I114 can flow through the resistors 114 and 118, and can be given by I114=V114/R114=I110*R110/R114, where R114 represents resistance of the resistor 114. The sense voltage V118 on the resistor 118 therefore can be given by V118=I114*R118=I110*R118*R110/R114, where R118 represents resistance of the resistor 118.
As a result, the sense voltage V118 is proportional to the charging current I110. The DC/DC converter 102 can receive the sense voltage V118 and adjust the charging current I110 to a desirable current level according to the sense voltage V118. However, usually the sense resistor 110 is relatively expensive. In addition, the operational amplifier 112 may cause relatively high power consumption.